Hitherto, there is an additional data-embedding technology where additional information is superimposed in an image to embed the information. Recently, use of the additional data-embedding technology in protection of copy rights (e.g., prevention of illegal copy) of digital works, such as a still image has been actively performed. As one example of the use of the additional data-embedding technology, there is a technology where a so-called invisible pattern which is an image that is difficult to visually recognize is formed on a recording medium together with a digital work, when the digital work is photocopied by an image forming apparatus, to thereby embed information related to the image forming apparatus.
As a method for reading the invisible pattern, infrared ray absorption has been used. For example, proposed is that an image formed with a general color toner and an image formed with a colorless toner including an infrared ray-absorbing material (may be referred to as an “invisible toner”) are formed parallel or with being superimposed, and images are recorded in a manner that the two image regions are substantially impossible to distinguish or difficult to distinguish with naked eyes (see, for example, PTL 1).
Moreover, proposed is that, when glossiness of an invisible toner is made lower than glossiness of a color toner and a color toner image formed in the same region to an invisible toner image on a surface of an image output medium is visually observed, information can be recorded with high density in the invisible toner without impairing image quality of the color toner image, and the invisible toner image with which mechanical reading and a decoding process by infrared light irradiation is performed stably over a long period can be obtained (see, for example, PTL 2, PTL 8, and PTL 4).